In a conventional papermaking machine wherein a wet web is transferred in open draw, that is, the wet web is transferred without a support, the web is susceptible to rupture at the transfer points. The possibility of rupture places a limitation on the speed at which the papermaking machine may be run. Consequently, closed draw papermaking machines, i.e. machines in which the web is transferred with a support, are becoming more prevalent. The closed draw machines can be operated stably at higher speeds.
In a conventional closed draw papermaking machine, a pickup felt picks up a wet web from a preceding section of the machine, holding the web underneath the bottom surface of the felt and transferring it to a next stage transfer belt.
The wet web is then pinched between the pickup felt and a transfer belt at a first nip formed between a pair of press rolls. Water is squeezed out of the wet web at the first nip. The wet web is then released from the pickup felt and transferred to the transfer belt. The transfer belt carries the wet web to a second nip formed between a third press roll and a shoe press, where further water is drained from the web and picked up by a press felt.
The wet web remains on the transfer belt until it reaches a vacuum roll, where the web is transferred to a canvas belt for heating and drying.
The closed draw papermaking apparatus utilizes a number of transfer points. Therefore, it is important for the wet web to be released easily to avoid maloperation. It is especially important for the wet web to be released easily at the location immediately following the shoe press because, if the transfer belt is smooth, a continuous film of water is likely to be formed between the wet web and the transfer belt. The water film can cause the wet web to adhere so tightly to the transfer belt that it cannot be released, even by the force applied by the vacuum roll.
The problem of web releasability has been addressed by utilizing the technology disclosed in Japanese published unexamined patent applications 57678/1994 and 88193/1985.
In the case of application 57678/1994 a filler is mixed into a synthetic resin layer formed on the surface of a base layer. The synthetic resin layer is ground after the resin has cured, and the filler forms projections on its surface. These projections impart a roughness to the surface and are very effective in breaking up the water film formed between the web and the transfer belt.
In the case of application 88193/1985, a synthetic resin layer 32 is applied to the surface of a base layer 31 by spraying in such a way that numerous air bubbles are formed in the synthetic resin layer. Upon grinding of the synthetic resin layer after it is cured, the air bubbles at the surface produce a multitude of concavities. These concavities impart a roughness to the surface of the belt which is effective to break the water film between the transfer belt and a wet web carried thereby.
The projections and concavities on the surfaces of the belts of the prior art can be formed only by grinding the surface of the synthetic resin layer after it has been cured. The need for curing and grinding results in a high production cost. In the case of the prior art belt utilizing a filler, because of the hardness of the filler particles, the projections are likely to impart markings to the wet web. In both cases, the prior art belts, which are composed of a base layer and a synthetic resin layer, tend to be heavy and difficult to install in a papermaking machine, depending on the structure and surroundings of the machine.
This invention addresses the above-mentioned problems. An important object of the invention, therefore, is to provide a wet web transfer belt in which a rough surface, permitting easy release of a wet web at transfer points in a closed draw papermaking machine, can be produced easily. Still another object of the invention is to enable the production of a wet web having a rough surface of reliable quality. It is also an object of the invention to produce, on a wet web transfer belt, a rough surface which does not produce markings on the wet web.